1. Field of the Presently Disclosed and Claimed Inventive Concepts
The presently disclosed and claimed inventive concept(s) relates generally to polyethylene terephthalate (“PET”) polymers, digested oligomeric derivatives of PET (“dPET”), functionalized oligomeric derivatives of dPET (“dfPET”), and polyurethane dispersions (“PUD”) made from or incorporating dfPET therein. More particularly, but not to be construed as limiting, the presently disclosed and claimed inventive concept(s) relate to lower molecular weight functionalized digested PET materials (“dfPET”) made from digesting polyethylene terephthalate, especially recycled polyethylene terephthalate (“rPET”). In one particular aspect, the presently disclosed and claimed inventive concept(s) relate to the production of an oligomeric form of functionalized digested polyethylene terephthalic acid from waste products, such as beverage containers, made from polyethylene terephthalate. In one embodiment, the dfPET polymers have an average number molecular weights of from about 200 to about 2000 Daltons. These dfPET polymers have excellent solubility in various organic solvents and provide a functionalized backbone for the production of polymeric based products such as polyurethane dispersions (PUDs) and polyurethane resins (PURs), by way of example but not by way of limitation.
2. Background and Applicable Aspects of the Presently Disclosed and Claimed Inventive Concept(s)
Most polyester resins used in commercial applications are formed from raw materials, which are rising in price and have relatively large markets. Accordingly, recovery of these raw materials from scrap, waste and used products is an important economical consideration as well as an ecological consideration. One widely used polyester is polyethylene terephthalate (hereinafter “PET”) made from terephthalic acid and ethylene glycol. Additionally, a Bisphenol A polyester resin could be used in a manner similar to PET. It should be understood that one of ordinary skill in the art, given the present disclosure and teachings, would be fully capable of using the presently disclosed and claimed inventive concept(s) to break down or degrade any polymer having a polyester backbone (e.g., polycarbonates) into oligomeric forms for use as a precursor molecule in the process of synthesizing polyurethane dispersions and resins.
Over the past 20 years, there has been an increased push throughout the world to increase recycling of polyester resins. Plastic bottles commonly used for drinks and carbonated beverages, for example, are made from polyethylene terephthalate and represent a large potential source of recoverable polyesters: either as bulk refined PET or the terephthalic acid and ethylene glycol monomers that constitute PET. It is estimated that from 375 to 500 million pounds of polyethylene terephthalate were used for beverage bottles in 1980, for example. More recently, more than 2.4 billion pounds of plastic bottles were recycled in 2008. Although the amount of plastic bottles recycled in the U.S. has grown every year since 1990, the actual recycling rate remains steady at around 27 percent. Recent legislation in several states requiring a deposit refundable upon return of all empty beverage containers has established an ongoing procedure for collecting and separating polyethylene terephthalate containers which must be recycled or otherwise disposed of in an economical manner. Additionally, many municipalities have implemented voluntary or mandatory recycling programs in conjunction with trash pickup and disposal.
PET beverage containers cannot be reused since the elevated temperatures required for sterilization deforms the container. PET containers can, however, be ground into small pieces for use as a filler material or remelted for formation of different articles. Such recycled material may be referred to interchangeably herein as “recycled PET”, “scrap PET”, “waste PET”, and/or “rPET”. The polyethylene terephthalate recovered by such processes contains impurities, such as pigment, paper, other undesirable polymers and metal from caps. Consequently, applications for polyethylene terephthalate reclamation by mechanical means are limited to non-food uses and low purity molded products.
In the past, several different techniques have been proposed for recovering pure or isolated terephthalic acid and ethylene glycol monomers from waste polyethylene terephthalate. One known technique involves, for example, the depolymerization of polyethylene terephthalate by saponification. Saponification is the hydrolysis of an ester under basic conditions to form an alcohol and the salt of a carboxylic acid (carboxylates).
In one known approach for saponification, polyethylene terephthalate is reacted with an aliphatic alcohol and a dialkyl terephthalate is recovered. This approach is exemplified in U.S. Pat. Nos. 3,321,510, 3,403,115 and 3,501,420, all of which are hereby incorporated by reference in their entirety. In a second known approach, polyethylene terephthalate is reacted with an aqueous solution of an alkali metal hydroxide or carbonate (usually sodium hydroxide) at an elevated temperature to yield a water soluble salt of terephthalic acid and ethylene glycol. The reaction product is acidified to liberate terephthalic acid which is water insoluble and the terephthalic acid precipitate is separated by filtration or the like. This approach is exemplified by U.S. Pat. Nos. 3,377,519, 3,801,273 and 3,956,088, all of which are hereby incorporated by reference in their entirety. U.S. Pat. No. 3,544,622, the entire contents of which is hereby incorporated by reference in its entirety, similarly discloses a variation to previously known approaches wherein the reaction is carried out under conditions to produce a water insoluble salt of terephthalic acid which is separated, washed and then acidified to produce terephthalic acid. Additional patents have also been issued on various improvements to these processes, such as U.S. Pat. Nos. 5,414,107, 5,223,544, 5,328,982, 5,045,122, 5,710,315, 5,532,404, 6,649,792, 6,723,873, 6,255,547, 6,580,005, 6,075,163, 7,173,150, 6,770,680, 7,098,299, and 7,338,981, the entire contents of each of which are hereby incorporated by reference in their entirety.
Empty beverage containers obtained from consumers may have aluminum caps lined with polyvinyl chloride or the like, wrap around polypropylene coated paper labels bonded to the surface with a polyvinyl acetate adhesive, residual sugars and, in some cases, polyethylene base caps for strengthening purposes. Without costly controls, reaction conditions in the saponification processes disclosed in the above-noted patents tend to cause some dissolution of these extraneous materials which then become impurities in the recovered terephthalic acid and require costly purification. Therefore, various approaches have been considered for removing these materials from the containers prior to grinding or separating them from the polyethylene terephthalate after grinding. Such separation procedures represent a significant increase in the overall cost of recovery as well as an energy inefficient means of recycling the waste PET. Thus, while such saponification methodologies for the recycling of PET into its monomer constituents are generally considered to be successful, it is an expensive and economically inefficient way in which to obtain such monomers for producing new PET polymers for use.
Polyurethane dispersions are used in a range of coatings as film formers or binders including adhesives, as well as other technical products. Polyurethane dispersions are considered an environmentally friendly alternative to solvent-based binders and they are increasing in importance in the manufacturing sector. The general advantages of polyurethane plastics are their flexibility at low temperatures, selectable mechanical properties, resistance to certain chemicals and, depending on the structure, resistance to weathering and environmental degradation. In order to achieve sufficient commercially desirable properties, high molecular polymer weights are required. A polyurethane dispersion is advantageous in that even at very high molecular weights the viscosity is determined mainly by the particle size of the dispersed resin in the dispersion. Polyurethane dispersions are, therefore, a solution for replacement of organic solvent based polyurethane coatings with regard to environmental considerations. For these reasons, polyurethane dispersions have become highly desirable for replacement products for organic solvent based polyurethane coating.
Polyurethane dispersions typically have a fairly broad distribution of different particle sizes in solution. Generally, polyurethane particles found in a stable dispersion are spherical in shape and are in a size range between about 30 nm and 1000 nm, and have a milky white (sometimes yellowish) appearance. Particles below about 50 nm in size result in a dispersion appearing increasingly transparent, while dispersions having particle sizes above about 1000 nm tend to settle out of solution and generally such dispersions are not storable for extended periods of time.
The percentage by weight (solids) of the polyurethane compounds in commercial polyurethane dispersions typically ranges from about 25 to about 50% weight percent and, in some cases, up to 60% by weight. Polyurethane dispersions having a high solids content are advantageous in terms of transport and storage, increased dry film thickness in a single application, and drying effective mass per share (as less energy is spent for evaporation of the water). While polyurethane dispersions having an increased solids content are preferable, in the past their production and use have been difficult and many compositions that appeared initially useful, oftentimes failed after commercial implementation.
Polyurethanes generally have a density, although specifically dependent upon their composition, of about 1.1 g/ml and are therefore heavier than water. The tendency for polyurethanes to settle out of solution and/or coagulate is typically prevented by mutual repulsion of the particles (e.g., internal ionic groups), and/or by the viscosity of the liquid dispersant. Polyurethane dispersions may, therefore, include thickeners and emulsifiers in the aqueous phase in order to retard settling and increase storability. Non-ionic stabilization occurs through, for example, the (i) incorporation of hydrophilic polyethylene oxide chains within the polymer chain or as terminal groups, (ii) an ionic stabilization through the incorporation of anionic groups such as carboxy or sulfonate groups, and/or (iii) incorporation of cationic group such as aminies. Non-ionic, anionic and cationic polyurethane dispersions can, therefore, be made depending upon application and starting material.
In addition to water, polyurethane dispersions may also contain a water-dilutable, high-boiling organic solvent/cosolvent (e.g., N-methylpyrrolidone (NMP)), as well as glycol ethers. The use of organic co-solvents allows, in some instances, the formation of hard polyurethane films at room temperature by partially dissolving the surface of the dispersion by evaporation of water and subsequent merger into a film (i.e., coalescence). The co-solvent evaporates, without any further heat treatment, and the film becomes harder and reaches its final strength. The co-solvents contribute to the emission of organic components (VOCs) and are, therefore, less desirable than fully aqueous dispersions. As an alternative to NMP, N-ethyl-2-pyrrolidone (NEP) may be used. In order that a desired high molecular weight polyurethane dispersion is produced, it is known in the art that the polyurethane polymers are preferably linear in structure—i.e., the polyurethane has very little branching in the polymer structure. Highly branched polyurethanes result in polymers that gel thereby hindering subsequent film formation.
The basic building blocks of a polyurethane dispersion consist, therefore, of bifunctional subunits that react to form substantially linear polymer chains. These polymer chains are generally similar to those of well-known polyurethane and are constructed of similarly known components—i.e., isocyanates, polyols, and polyamines. Depending on the isocyanates used, a distinction can be made between aliphatic and aromatic polyurethane dispersions. The latter are generally less expensive to produce, but have the disadvantage of yellowing when exposed to light (with the exception of tetramethyl xylylene diisocyanate (TMXDI)). The fraction of incorporated isocyanate in a polyurethane dispersion is generally less than about 20% by weight, and more generally from about 8 to about 12% by weight.
Polyols (including the dfPET of the presently disclosed and claimed inventive concept(s)) are typically the largest mass fraction of the polyurethane compound. Through selection of a polyol with a correspondingly low glass transition temperature, a polyurethane can be generated with a corresponding low-temperature flexibility and have two or more hydroxyl groups at the terminal ends. The structure of the polyurethane polymer generally proceeds in two steps: first, a branched prepolymer is prepared from a combination of diisocyanates and polyols (for example, the dfPET of the presently disclosed and claimed inventive concept(s)). By using an excess of diisocyanate, the prepolymers are formed having isocyanates terminal groups. In the second step, the prepolymers (e.g., short-chain diols and/or diamines) are linked to longer-chain molecules during dispersion. Such dispersions often occur in the presence of water and/or include an organic solvent as a cosolvent.
The chain extender composition (which can be water in certain instances) can incorporate ionic groups into the polymer and thereby stabilize the water-dispersed polyurethane particles. A typical chain extender composition is dimethylol propionic acid (DMPA) which adds carboxy-functionality, while diolsulfonate can be used to add sulfonic acid groups useful for anionic polyurethane dispersions. With regard to anionic polyurethane dispersion, isocyanates and polyols are reacted to form a prepolymer. Via chain extension, a hydrophilic group is added—e.g., a diamine with pendant sulfonate. The resulting polyurethane polymer is permanently hydrophilic and can be dispersed in an aqueous medium. For the preparation of cationic dispersions, quaternary amino functional groups may be incorporated into the polyurethane polymer using N-methyldiethanolamine (NMDEA), for example. As would be appreciated by one of ordinary skill in the art, there is a nearly endless list of variations that can be made to the polyurethane to incorporate different functional groups with variable functionality. The introduction of terminal, blocked isocyanates improves heat-activatable crosslinking reactions, for example. Additionally, the addition of epoxy groups and silane functionality into the polyurethanes is desirous, in some products, while pendant hydroxyl groups incorporated into the polyurethane polymer is desirous for crosslinking other reactive agents, such as those used in coating formulations.
Generally, it has been believed in the art that the dispersion of polyurethanes in water required high shear forces in order to obtain a corresponding finely dispersed product. This has been due to the high viscosity of the isocyanate prepolymer. Further, after chain extension the polyurethane polymers are even less dispersible in water. Therefore, there have been developed two traditional approaches to creating the finely dispersed products: (a) the prepolymer is dispersed directly in water and chain extension occurs under high shear forces and the chain extension takes place in the presence of the aqueous phase. In order to further lower the viscosity of the prepolymer, it can be heated prior to or during dispersion into the aqueous medium. Additionally, a co-solvent can be used to lower the viscosity of the prepolymer (e.g., acetone). The co-solvent will, however, generally remain in the finished dispersion and the co-solvent must be carefully chosen in order that it not interfere with the drying and curing of any coating composition made from the resulting polyurethane dispersion. (b) The complete polyurethane molecule is built up in a water-immiscible low-boiling solvent. The solution is dispersed with water, and the solvent is distilled or otherwise removed. As the solvent of choice is generally acetone in this method, this second process is referred to as the “Acetone Process.” The advantage of the Acetone Process is that it has a high variability of starting materials that can be used as well as the absence of organic solvents. The disadvantage is the lower boiler efficiency compared to other techniques and the increased cost of acetone recovery. Alternatively, some have replaced the acetone with 2-butanone (MEK).
Typical applications of polyurethane dispersions include planar applications that allow the water to evaporate and, optionally, any cosolvents are able to leave the resulting polyurethane film. The drying of the polyurethane dispersion made films can occur at room temperature or at elevated temperatures, if permitted by the substrate. Once the dispersion and substrate are in contact, when enough water has evaporated the operation is not reversible (coagulation). In the gaps between the polyurethane particles dispersed on the substrate, high capillary forces ensures that the polyurethane particles lose their phase boundaries, fuse together (coalescence), and form a homogeneous film after drying. Cosolvents may be used to increase and/or inhibit the time to coalescence.